At present there are numerous metal detecting devices having transmitters and receivers duly adjusted to emit and receive signals which, through the emission of a magnetic field, receive the corresponding signals of the magnetic material sought to be detected. However, they are essentially dedicated to the same type of magnetic material, without any possibility of detecting those that, owing to their different hysteresis cycles, are beyond the reach of the detector used.
An example of a prior art system which has a frequency synthesizer is found in U.S. Pat. No. 4,303,885. The frequency synthesizer is responsive to digital control signals applied along a control buss to generate a square wave at a frequency determined by the digital command. The square wave is filtered into sinusoidal signals which is sent to excite coils.
In order to eliminate this inconvenience and to distinguish different magnetic materials with different hysteresis cycles, the system of the present invention has been developed. This system utilizes the emission of two different frequency magnetic fields at proper amplitude. As a result, it is possible to operate in the non-linear area of the hysteresis cycle to produce a frequency beat to obtain, depending on the actual material hysteresis cycle and additional factors, an amplitude of the product obtained by the above frequency beat.
A further survey of the amplitude and its relationship to the product will distinguish the different materials detected with different hysteresis cycles.
Consequently the system comprises a transmitter, a receiver and an electronic assembly. The transmitter is formed with one or more coils in arrangement and number depending on the application. The receiver has one or more pairs of coils and the electronic circuit has a microprocessor which provides the signal from which the remaining required signals are generated. At the same time the microprocessor processes the signal received and the result of this process is provided as an output to an external indicator.
Of the above signals, two of them are conveniently filtered and amplified and are utilized to provide to the transmitting coils the current required for the emission of the previously established magnetic fields. The other signal controls the different central or cut-off frequencies of the different filters. In these filters, the signals received by the antenna are processed, amplified and filtered in such a way that, when they are transferred to the microprocessor, the analysis thereof is produced and the corresponding result is provided on an external indicator.
This system therefore allows the operations that the existing systems cannot perform, such as: the possibility of detecting materials containing magnetically soft materials such as permalloy, micro-metal, metals, etc. with low transmission frequencies; and choosing among materials with close values to study the products providing signal amplitude with these values, which allows the reduction of the field by limiting the materials to be detected to those presenting different hysteresis cycles. Below, a detailed description of the material detecting system is provided, with reference to an illustrative embodiment, subject to all those detailed modifications which do not fundamentally alter the essential characteristics thereof, as defined in the claims.